Loudspeaker



April 1952 J. D. SPRAGINS, JR, ETAL 3,027,964

LOUDSPEAKER Filed June 24, 1958 Q Z W W M.

i E 1 r JOHN D. SPIQAGINS JR. LEONAQDT POCKMAM GEORGE A. BRETTELL lNVE025 W ATTORNEYS United States Patent 3,027,964 LOUDSPEAKER John D.Spragins, Jr., Palo Alto, and Leonard T. Pockman, East Palo Alto,Calif., and George A. Brettell, Mexico City, Mexico, assignors to AmpexCorporation, Redwood City, Calili, a corporation of California FiledJune 24, 1958, Ser. No. 744,227 7 Claims. (Cl. 181-81) This inventionrelates generally to loudspeakers and more particularly to wide anglehorn loudspeakers.

As is well known, horn-loudspeakers are very eflicient for soundreproduction. This is due, in most part, to the ability of the horn topresent almost any desired value of acoustical impedance to a generator.As a result, it is possible to achieve maximum overall design of a soundsystem.

A major problem in designing loudspeakers is that of obtaining a uniformdistribution of sound power over a large solid'angle for a relativelywide range of frequencies. At low frequencies, little diificulty isexperienced in obtaining a uniform sound pattern. However, at the higherfrequencies, speakers become'directional and the directionality isdependent upon the wavelength of the sound. The foregoing is notsurprising when it is considered that the audible range includeswavelengths which vary between about 0.6 inch and 50 ft.

In general, loudspeakers are designed for optimum performance at themid-frequency range. At very low frequencies, the speaker acts as apoint source with a uniform radiation pattern. At wavelengths near thelargest dimension of the speaker, the radiated sound becomes highlydirective and in most cases stays directional as the frequency isfurther increased. A number of effects combine to cause thisdirectionality. Typical ones are: different path length for sound fromdifferent parts of the speaker, phase differences between differentparts of the speaker diaphragm, andtransverse vibrations at the mouth.

Attempts to eliminate the directional effect involve creating roughlyspherical wave fronts leaving the speaker. Several schemes have beentried in the prior art. A common method is to form a multi-cellular hornin which the horns are arranged in columns and rows with the hornshaving a common throat and the mouths forming a portion of a sphericalsurface. However, horns of this type have a wide variation indirectivity as the frequency is varied.

The directional characteristics of these horns may be explained fairlysuccessfully as the pattern of one large horn at low frequencies and asthe sum of the radiation from several highly directive horns at higherfrequencies. Generally, a speaker of this type radiates power uniformlyover a wide angle only when the dimensions of its surface are large incomparison to the wavelength being radiated.

It is a general object of the present invention to provide a novelloudspeaker which is relatively smalland which radiates power uniformlyover a large solid angle.

It is another object of the present invention to provide amul-ti-segment loudspeaker in which the sound is caused to arrive inphase at the mouth of all the segments by including velocity determiningmeans in each of said segments.

It is another object of the present invention to provide a hornloudspeaker which employs a plurality of concentric horns forming aplurality of horn segments terminating on a portion of a spherical orarcuate surface with means included in said horns for determining thevelocity in each segment.

It is another object of the present invention to provide a hornloudspeaker which employs a plurality of concentric hor'ns forming aplurality of horn segments te minating on a spherical surface with amixture of gas included in predetermined ones of said horn segments fordetermining the velocity of the sound within the same whereby the soundarrives in phase at the mouth of the various segments.

These and other objects of the invention will become more clearlyapparent from the following description when taken in conjunction withthe accompanying drawings.

Referring to the drawing:

FIGURE 1 is a perspective view showing a loudspeaker in accordance withthe invention;

FIGURE 2 is a sectional view of the loudspeaker of FIGURE 1; and

FIGURE 3 shows construction lines for forming a loudspeaker inaccordance with the invention.

Referring to FIGURE 1, the loudspeaker comprises a plurality ofconcentric horns 11, 12, 13, 14, 15 and 16. The space between the hornsis a plurality of concentric segments. A driver 18 serves to drive thespeakers at the throat 19. Each of the horns 11-16 is a smoothlyexpanding horn which terminates normal to a substantially sphericalsurface. The horns expand in such a manner that the ratio of areas ofeach of the horn seg ments is the same along the axis of theloudspeaker, as for example, at the planes 21 and 22, in FIGURE 2.

Any suitable driver, for example, the driver shown in FIGURE 2 may beemployed for driving the horn. The driver illustrated includes amagnetic assembly 23, a voice coil 24' immersed in the magnetic field atthe gap 25, and a diaphragm 26 which feeds into the horn segments 27 and28 illustrated. The concentric horn assembly is suitably secured to thedriver, as for example, by screws 31.

A gas-tight diaphragm 32 is suitably sealed betweenthe outer horn 16 andthe driver and is suitably sealed The corrugations allow for expansionand contraction during atmospheric pressure variations.

Thus, it is seen that the segments form a plurality of enclosures intowhich mixtures of gases may be introduced for purposes to be presentlydescribed. Preferably, the Mylar films are treated whereby the gasesenclosed within the segments will not permeate outwardly. For raregases, the Mylar film may he made impervious by coating it with arelatively thin film metal, as for example by evaporation.

Thus, it is seen that a speaker is provided in which the mouths of thevarious horns terminate on a spherical surface. If the sound arrives inphase at the mouth of each of the segments, a nearly hemispherical wavefront will be generated going out away from the speaker. However, it isseen that unless means are provided for changing the velocity of soundin the various horn segments, it will arrive out of phase at each of themouths since the path length for the sound is different in each of thehorn segments. For example, in the illustration, the path length fromthe driver to the outer annular mouth is about of the path length fromthe driver to the mouth of the inner horn. Suitable means are providedfor changing the velocity of sound so that it arrives in phase. Forexample, an acoustical delay line may be introduced. Preferably, theloudspeaker is formed with the gas-tight diaphragms at the two ends andmixtures of different gases are introduced into the segments. As is Wellknown, the velocity of sound in various gases is different beingapproximately'inversely proportional to the square root of the molecularweight.

In one particular example, helium was employed in which the velocity ofsound is approximately three times that of the velocity in air. Bymixing appropriate amounts of helium and air, any intermediate velocitycould be realized. Helium being a rare gas did not permeate through thecoated Mylar film. By employing helium mixed with air and using aslittle helium as possible in the inner sections, it was possible tominimize reflections of the sound at the mouths. In the illustrativeexample, it is noted that the outer horn contains pure air with theinner horns being arranged to contain mixtures of helium and air to givethe appropriate velocity, Thus, relatively small amounts of helium wereadded and the reflections introduced were at a minimum.

It is, of course, to be understood that the Mylar film might enclose allof the horn segments and that other gases might be employed forcontrolling the velocity. However, because helium has such a largechange in velocity relative to air, it is preferable since relativelysmall amounts need be added.

As previously described, the ratio of areas of the segments for allpoints along the axis must be substantially equal and the hornspreferably should terminate on a spherical surface. True exponentialhorns have only one parameter which may be varied, while two parametersneed be varied to satisfy these conditions. Thus, a more general form ofhorn equation need be employed to obtain the desired variation. Theequation may be used to generate an ordinary exponential horn, acatenoidal horn, a conical born, or any other interediate shapes. Theequation involves three parameters: y the radius of the throat, which isvirtually fixed by the size of the driver, i the cut-off frequency forthe horn which is limited by the cross-over frequency of the speaker,and T which represents the shape factor and can be varied with completefreedom. However, to design concentric horns which comply with thisequation would be rather ditlicult with the many limitations which areplaced on the parameters. Consequently, a modified type of concentrichorn which terminates normally to a spherical surface and which has theproper area ratio was constructed. v Referring to FIGURE 3, a hornsimilar to that of FIG- URE 2 with the addition of construction lines isillustrated. In construction, circular arcs 41 are drawn with theircenters on the axis of symmetry of the horn. The circles are drawnaccording to calculations from the foregoing equation for the innerhorn. The outer horns are constructed so as to intersect all the circlesat very nearly right angles. The ends of the horns terminate on aspherical surface and the ratio of areas is preserved throughout.

The horns, as designed in accordance with the foregoing, are only anapproximation to the horn defined by the foregoing equation with theouter horns deviating an appreciable amount. However, mathematicalexpression for horns usually break-down when end effects and othereffects are considered. Thus, almost any smoothly expanding horn willperform virtually as well as a true mathematical horn and the aboveillustration is given only for teaching how a horn might be formed tocomply with the invention.

Thus, it is seen that an improved loudspeaker is provided. The horn isrelatively small since it includes a plurality of concentric hornsegments having their mouths terminating on a spherical surface. Thehorns are designed whereby they have a substantially equal ratio ofareas along the axis. Means are provided for varying the velocity of thesound within the same whereby it arrives in phase at the mouths whichlie at different path lengths from the driver. The loudspeaker providesa substantially spherical output wave.

We claim:

1. A loudspeaker comprising:

a plurality of horns arranged with their mouths terminating on a surfaceto form a common mouth, each of said horns having a difierent pathlength; and

means comprising different predetermined mixtures of gases included ineach of said horns for controlling the sound velocity therein so thateach sound wave introduced at the smaller ends of said horns arrives atthe common mouth substantially in phase.

2. A loudspeaker comprising:

a plurality of horns arranged with their mouths terminating on a portionof an arcuate surface to form a common mouth, each of said horns havinga different path length; and

means comprising different predetermined mixtures of gases included ineach of said horns for controlling the sound velocity therein so thateach sound wave introduced at the smaller ends of said horns arrives atall parts of the common mouth substantially in phase.

3. A loudspeaker comprising:

a plurality of concentric horns arranged with their mouths terminatingon a common surface to form a common mouth;

said horns defining a plurality of concentric horn chambers, each of adifferent path length; and

means comprising different predetermined mixtures of gases included ineach of said horn chambers for controlling the sound velocity therein sothat each sound wave introduced at the smaller ends of said hornsarrives at all parts of said common surface substantially in phase.

4. A loudspeaker comprising:

a plurality of concentric horns arranged with their mouths terminatingon a spherical surface to form a common spherical mouth;

said horns defining a plurality of concentric horn chambers, each of adifferent path length; and

means comprising different predetermined mixtures of gases included ineach of said horn chambers for controlling the sound velocity therein sothat each sound wave introduced at the smaller ends of said hornsarrives at all parts of said common surface substantially in phase.

5. A loudspeaker comprising:

a plurality of smoothly expanding concentric horns arranged with theirmouths terminating on a spherical surface to form a common sphericalmonth;

said horns defining a plurality of concentric horn chambcrs, each havinga throat and being of a different path length; and

different mixtures of gases included in each of sai horn chambers forcontrolling the sound velocity therein, the mixture of gases in eachchamber being selected so that each sound wave introduced at the threatsof said horn chambers arrives at all parts of said common mouthsubstantially in phase;

whereby sound is radiated from said spherical surface with substantiallyspherical Wave fronts.

6. A loudspeaker comprising:

a plurality of smoothly expanding concentric horns arranged With theirmouths terminating on a spherical surface to form a common sphericalmonth;

said horns defining a plurality of concentric horn chambers, each of adifferent path length;

said chambers having a constant ratio of areas for all points along theaxis of said horns; and

different mixtures of gases included in each of said horn chambers forcontrolling the sound velocity therein, said mixtures being selected sothat each sound wave introduced at the throats of said horn chambers ar-1,840,992 Weitling Jan. 12, 1932 rives at all parts of said common mouthsubstantially 1,992,268 Wente Feb. 26, 1935 in phase. 2,001,089 BlattnerMay 14, 1935 7. A loudspeaker as characterized in claim 6, wherein:2,037,187 Wente Apr, 14, 1936 each of said horns terminates normally onthe spherical 5 2,797,766 ullivan July 2, 1957 surface 2,819,771 KockJan. 14, 1958 I 5 References Cited in the file of this patent 2820525Fountam et M Jan 21 19 8 UNITED STATES PATENTS FOREIGN PATENTS 1,770,234Grant July 8, 19 30 473,046 France Sept. 5, 1914 1,801,521 Milnor Apr,21, 1931 372,136 Germany Mar. 20, 1923

